This invention relates to a sublimer assembly for subliming particulates. It is particularly useful in Kroll reduction processes for vaporizing zirconium tetrachloride, uranium fluoride and the like.
Sublimer assemblies are generally employed in commercial processes to vaporize solids to directly produce vapors for reacting with molten magnesium or sodium. For example, zirconium tetrachloride and hafnium tetrachloride in particulate form are sublimed in commercial Kroll processes so that vapors react with liquid magnesium to form a metal sponge and liquid magnesium chloride. See, e.g., U.S. Pat. No. 4,511,399 which describes sublimers for vaporizing zirconium tetrachloride.
U.S. Pat. No. 4,511,399 describes a production sublimer assembly comprising a retort welded to a lower crucible. The retort generally has a substantially vertical sidewall and a concentric downcomer extending through a bottom wall into the lower crucible. Zirconium tetrachloride particulate feed in the annular volume defined by the sidewall and the downcomer vaporizes, flows down through the downcomer and reacts with liquid magnesium in the lower crucible to form a reduced zirconium sponge in a liquid magnesium chloride pool. The heat to vaporize the zirconium tetrachloride may be provided by a heating means (which is not shown but may be an induction coil of an electric furnace adjacent the sidewall of the retort similar to the coil shown in the patent surrounding the lower crucible or a natural gas furnace). In a typical production cycle, thousands of pounds of zirconium tetrachloride are batch fed into a generally cylindrical retort having a diameter of up to 6 feet or more. The feed may fill the entire annulus between the downcomer and the sidewall up to the top of the downcomer. Typically the height of the particulates exceeds the diameter of the retort. The feed is then heated to at least about 370.degree. C. and vaporized, and a frangible seal in the downcomer is broken to allow the vapors to flow through the downcomer and into the crucible. The feed continues to vaporize at about 370.degree. C. as the reaction in the crucible proceeds at about 850.degree. C.
In practice, the Kroll reduction of zirconium tetrachloride in production facilities requires a long time. In addition, the vaporization rate in the sublimer retort tends to vary substantially during cycles in which the rate is initially high, falls off and then increases. If the cycling could be eliminated, or at least reduced, the cycle time would be reduced.
The applicants theorize that the vaporization rate in the sublimer assembly is heavily (but not entirely) dependent upon thermal conduction into the feed adjacent the retort sidewall. Heat is conducted into the feed in accordance with the basic relationship (Q =UA(dt). The heat transfer coefficient and the temperature differential are reasonably constant in a sublimer. Thus, the principal variable is the heat transfer surface area between the sidewall and the feed. If the surface area changes drastically, then the vaporization rate also changes drastically. The vaporization rate of the feed (which is at about 370.degree. C.) is not uniform across its top surface during the cycle. Rather, the feed adjacent to the sidewall of the retort tends to vaporize first and thereby develop a cone shaped pile of particulate feed having substantially less physical contact with the sidewall. Thus there is substantially less thermal conduction and heat transfer. When the angle of the cone exceeds the angle of repose of the feed, the pile may slump to create a sudden increase in contact area between the retort sidewall and the feed so that the vaporization rate may substantially increase.
Toward the end of the process, the feed may not slump and there is minimal contact between the retort sidewall and the feed. Eventually the feed will only physically contact the retort bottom wall. The cycle time must therefore be greatly extended to vaporize the last portion of the feed.